1. Field of the Invention
The present invention relates to the field of communication systems in general and, in particular, to a method and apparatus for rejecting image signals in a receiver.
2. Background Information
The wireless communications industry has been swept up in a technological revolution over the past two decades. Commercial wireless systems have evolved from simple one-way (paging) communication systems to two-way communication systems comprising analog cellular systems and, more recently, digital cellular systems. During this technological revolution, the size and weight of the wireless communication handset has been dramatically reduced. Today, handsets that fit comfortably in the palm of one's hand and weighing less than seven ounces are commonplace. This dramatic reduction in size and weight has been facilitated by large scale integration of the many discrete components that historically comprised the electronics of the transceiver (e.g., transmitter and receiver), reducing the number of electronic components required to just a few. Thus, it is not surprising that the next logical step in this technological revolution is an integrated single-chip transceiver.
Those skilled in the art may well recognize that technical impediments remain before an integrated single-chip transceiver can be produced, reliably, in large quantities. In particular, integration of the discrete components comprising the receiver presents one of the greatest challenges. To understand these challenges, a brief review of a simple receiver is provided with reference to the block diagram of FIG. 1.
As depicted in FIG. 1, receiver 100 receives a carrier signal via input 102, in this case, coupled to an antenna. The received signal is passed through a low-noise amplifier 104, wherein the signal is filtered and amplified to further distinguish the desired signal from the noise floor. Downconverter 106 downconverts the received signal to an intermediate frequency (IF) signal, whereupon decoder (detector) 108 recovers the baseband signal containing the desired information from the IF signal. One example of a prior art downconverter employs one (heterodyne) or more (superheterodyne) mixers, which mix the received signal with a reference signal generated by a local oscillator. The resulting mixing of these two signals produce a "sum" and a "difference" of the two signals, wherein the sum component of the resultant is easily filtered out. However, (super)heterodyne downconversion employing mixers renders a receiver sensitive to an undesirable signal at 2.times. the IF from the desired carrier frequency, commonly referred to as the image signal. For example, suppose a receiver is tuned to 160 MHz, with an IF of 29.5 MHz, the receiver will thus be susceptible to image signals at a frequency of 160-(2.times.29.5)=101 MHz, a common FM-radio broadcast frequency. Thus, even though the receiver is "tuned" to receive and demodulate a signal at 160 MHz (e.g., the carrier signal), left unchecked, the downconverter will introduce an undesirable (image) signal at 101 MHz. Thus, one can see how the heterodyne downconversion process can subject the receiver to a number of image signals. Consequently, a high precision, high frequency filter (image-reject filter) is often employed prior to the mixing process to reduce the receiver's susceptibility to image signals. Similarly, a highly selective IF filter is required for the selection of the desired signal after downconversion. This filter is also difficult to implement using integrated filter technologies especially if the IF is relatively high.
Those skilled in the art of integrated circuit (IC) manufacturing will appreciate that while fabrication of low frequency passband filters within an IC is possible, the fabrication of a complex high frequency passband filter is extremely difficult on a large scale due to nominal process variations. Consequently, the prior art heterodyne downconversion technique does not lend itself to receiver integration.
Another category of prior art downconverters known as "image-reject mixers" involve the decomposition of the received signal into its in-phase and quadrature components, commonly referred to as quadrature downconversion. One example of a prior art quadrature downconverter is depicted in FIG. 2. As illustrated in FIG. 2, quadrature downconverter 200 splits the received signal down two parallel paths 202 and 204, respectively. In path 202, the received signal is mixed with a reference signal from a local oscillator (LO) 208 at mixer 206 to produce the in-phase (I), or real component. Along path 204, the received signal is mixed with the reference at mixer 212, wherein the reference signal from LO 208 is phase shifted by ninety degrees at phase shifter 210, producing the quadrature (Q), or imaginary component of the received signal. The in-phase component (I) and the quadrature component (Q) are then combined in a way to produce constructive interference on the signal of interest and destructive interference on the unwanted image signal.
Although quadrature downconversion can improve the image rejection of a receiver, the in-phase and quadrature signal processing paths of a quadrature downconverter must be very-well matched in terms of gain and phase over the frequency range of the local oscillator. Gain and/or phase imbalance will result in incomplete image signal suppression, and renders the receiver sensitive to unwanted image signals. Currently, practical mismatch in current IC manufacturing technologies is in the order of three-degrees (3.degree.), while gain mismatch is on the order of one to five percent (1-5%) which limits the image rejection of the receiver to no more than 30 dB. Consequently, quadrature image-reject mixers do not lend themselves well to current integrated technologies unless gain and phase compensation techniques are employed.
More recently, research on alternative downconversion techniques has focused on direct-conversion or zero-IF downconverter architectures. While disparate aspects of this architecture lend themselves well to integration technology by eliminating the need for selective RF and IF filters, they still suffer from many of the gain/phase matching problem discussed earlier, in addition to other problems arising from the fact that the desired signal is translated directly to DC, where offset and noise components may add to the desired signal. Consequently, none of the foregoing downconverter architectures are readily integrable into a large scale highly integrated single-chip transceiver.
Thus, a method and apparatus for rejecting image signals in a receiver is required that overcomes the inherent limitations and deficiencies commonly associated with the prior art. Just such a method and apparatus is presented in accordance with the teachings of the present invention that achieves these and other desired results.